Constraining Theia

The "Big Splat" theory has a few new constraints this week.

The regnant theory explaining our moon's formation is that two planets banged into each other about 50 million years into the Solar System's coalescence (UPDATE 7/13: or later). The debris re-coalesced into a heavily-iron Earth and a heavily-silicate Moon (UPDATE: constrained to 4.425 Gya UPDATE 10/24/23: 4.46?). The latter then suffered a dark green iron rain, locked its rotation with ours, and suffered further impactors; raising up lava seas on its side facing us which solidified into black basalt "maria".

Some problems with this: backtracking how far the Moon was at over time, it should have been within Earth's gravity well as of only one to two billion years ago. Well after the Splat theory's projections elsewhere. Also Earth and Moon look chemically similar - perhaps too similar. Where did the two protoplanet parents come from?

Now it is being projected that, of the two protoplanets, indeed the larger one was recognisably Earth, albeit rather lighter and larger (less dense, more sicila). Earth formed in-situ here like Venus had formed further-in. The smaller one was icier and (still) rockier - a Marslike. Forming about where Mars is now, perhaps, or in the Asteroid Belt. Definitely fair to give to a different planet its own name: "Theia" being the popular choice. Much of Earth's sicila ended up on her new Moon; some of Theia's heavy metals ended up here. But the Moon's mantle is mostly Theia's.

As for when: I don't know that there are surprises: our day was shorter then. But the rate of change, given a similar pattern of Earth's seas, is better-constrained. It was linear from the the late Cretaceous on UPDATE 6/12/23: but we need to backtrace before 2Gya.

IRON 02/02/24: Theia mantle 13-18% iron? Versus Earth 8% and Luna 10%. They did it with computers!!